1. Field of the Invention
The present invention relates to amplifier circuitry and, particularly to an amplifier circuitry having a controlled preamplifier stage and a power amplifier stage.
2. Description of Prior Art
The use of amplifier circuitry for supplying power (i.e., electrical energy) to drive a load is well known in the prior art. The load may be a loud speaker, a motor, or a motor connected to the supply and/or take-up reel of a tape transport unit.
Conventional amplifier circuitry includes one or more amplifier stages, for example, a preamplifier stage and a power amplifier stage. Usually the preamplifier stage processes a small analog signal while the power amplifier stage processes the analog signals outputted from the preamplifier stage.
One problem associated with prior art amplifier circuitry is that often times the particular application to which the amplifier is used requires that the amplifier performance exceeds the electrical rating of the components used to fabricate the amplifier. As is well known to those skilled in the art, electrical components such as transistors, diodes, resistors, etc., are assigned specific ratings as to voltage and/or current capabilities by the manufacturers. For example, the manufacturer will list a breakdown voltage for a transistor. This means that whenever a designer uses such a transistor in an amplifier the maximum voltage which can be applied across the transistor has to be at most equal to the breakdown voltage and, preferably, less than said voltage. Any attempt to use the transistor above the prescribed rating will result in catastrophic failure.
Due to the above limitation, designers are forced to take precautionary steps so as to avoid catastrophic failure. One approach used by the prior art to solve the above problem is to use components whose electrical rating as to voltage, current, etc., is below the requirement which a particular application imposes on said component. Generally, the higher the electrical rating on a particular component the higher will be the cost of said component and the subsequent cost of a product. Moreover, it appears to be a waste of resources when a device operates below its optimum rating.
Another approach used by the prior art to solve the above problem is the use of regulators for protecting a particular component. U.S. Pat. No. 3,486,124, issued to M. F. Eisenberg exemplifies the use of regulators as a regulating device. In the Eisenberg patent a pair of voltage regulators are used to control the voltage across the output transistors of a push-pull amplifier. The approach used is to maintain the voltage of the non operating transistor to be less than its optimum rating. This is done by distributing the voltage between the non operating transistor and the associated voltage regulator.
Although the use of regulators is an improvement over the prior art. This approach suffers from one major drawback, namely, the device (i.e., the power amplifier) requires a relatively large amount of energy to ensure proper operation. Of course, the consumed energy is not delivered to the load (i.e., probably it is not performing useful work). The primary reason for the high energy consumption is that the regulator is positioned in series with the nonoperating component. In order to distribute the voltage across the regulator and the nonoperating component, the regulator has to be "on" (that is in an active state). Needless to say, it behooves us to fabricate apparatus in which the energy consumption is minimized.